Perhaps 12GB in dual channel?

Question

RAM

There are two lines of 2 GB in dual-channel mode (2133 MHz), is it possible to transfer 2 to 4 GB with the same timings and frequency so that the dual-channel mode continues to work?

Answer 1

The so-called "gain" from the two-channel mode is 1-1.5%. Of course, the gain from the amount of RAM is much higher.
Better worry about the fact that the sub-timings of the RAM will not match.
Run Memtest overnight.

Answer 2

If you do not miss with slots for modules, then everything will work. The performance gain is highly dependent on the tasks being performed.